A nickel-hydride secondary battery using a hydrogen absorbing alloy, which can absorb and release hydrogen in a reversible manner, as an anode material has such features as low-pollution and high-energy density etc., so that it has been frequently used for power sources for portable equipment and electric motor etc., in place of a nickel-cadmium secondary battery and many studies and developments have been made thereon.
While, it is necessary to grind an alloy ingot into particles once and then to form them into a required shape, in order to make up an electrode by using the hydrogen absorbing alloy. For example, such methods are practiced industrially in general as (i) a method wherein alloy particles are added with binder to be formed into a paste and it is filled in a current collector and (ii) a method wherein it is formed into a sheet so as to be press bonded to the current collector, etc. In such electrode making-up process, a firm oxide film is apt to be produced on surfaces of the alloy particles. Since a hydrogen absorbing electrode on which surface the oxide film is produced offers an initial capacity fairly lower than an expected actual capacity, "activation" has been required at initial stage of charge/discharge. The activation means that the oxide film is destroyed to acquire an aimed discharge capacity by carrying out several cycles of charge/discharge at the initial stage of charge/discharge. In a battery using the hydrogen absorbing electrode having such a property, a discharge voltage and a battery capacity lower. Especially in a sealed-type battery, a capacity balance between anode and cathode is lost at the initial stage of charge/discharge, an inside pressure rise etc. occurs and a cycle life is reduced. In order to avoid the formation of oxide film, (i) a method for carrying out an electrode making-up process under an inert gas atmosphere and (ii) a method for putting an oxide film removal process in the electrode making-up process etc. are proposed. In these proposed methods, however, there have been such problems as a complex equipment and a troublesome work etc. In addition, even in the electrode prepared by the above proposed methods, there has been such a problem as the formation of oxide film depending on storage conditions after making-up.
Incidentally, in order to hasten the above activation, it is known that (1) the hydrogen absorbing alloy particles or hydrogen absorbing electrode is to be subjected to high-temperature alkaline treatment and (2) the hydrogen absorbing alloy particle is to be subjected to Ni-nonelectrolytic plating treatment. However, since insulating needle-like rare earth hydroxide is produced on the alloy surface in the treatment (1), it is necessary to carry out an ultra-sonic cleaning for removing the hydroxide. Further, a considerable rinsing work is required in order to remove alkali completely. Therefore, the treatment has been troublesome. The existence of needle-like rare earth hydroxide will cause a decrease in conductivity and a shortening of battery life. There are such problems in the treatment (2) that, (i) it is difficult to maintain constant plating conditions, (ii) handling is troublesome because an alloy after undergoing the treatment is inflammable, and (iii) the alloy is not continuous with a plated layer in term of grain structure so that, as the cycle goes on, the insulating needle-like rare earth hydroxide precipitates at a border part to cause a peeling-off of the plated layer.
In the nickel-hydride secondary battery, there were such problems as a rise of battery inside pressure at the last stage of charging to cause a breakage of container and scattering of electrolyte etc. The rise of battery inside pressure is said to be caused by hydrogen gas produced from an anode during charging. As a method for preventing such a rise of inside pressure, a method is known wherein Ni-plated layer is previously formed on the hydrogen absorbing alloy so as to enhance a hydrogen absorbing activity of the alloy. However, this method includes a problem of poor productivity due to a plating process which should be added to the electrode making-up process, besides the above problems similar to those of the above treatment (2).